Archiving and historical email

If I turn on archiving for an existing mailbox, what happens to the messages that are already in the mailbox? Will they get into the archive or is it only on a go-forward basis? Anyway to get the messages over to the archive via the command, or do I just have to do an imapsync or recovery or something?

Well from what I just tested out yesterday pertaining to archiving you are correct. It will begin archiving any messages that are sent after enabling ZAD (zimbra archiving and discovery) on the existing mailbox. Any emails previous to enabling ZAD are not stored... to my knowledge (feel free experts to prove me wrong) Sorry I don't have any answers for the other questions but I figured answering your first question was worth posting.

The archiving feature is a function of the MTA, so a piece of mail actually has to be transferred in order for it to make it into the archive account. Since the archive accounts are still normal accounts, you can do an imapsync to get the content into that account.